This invention relates to a rotational angle detecting apparatus for controlling an ignition timing of an internal combustion engine, and more particularly to an apparatus for generating pulse signals in synchronism with the rotation of a crankshaft of the internal combustion engine.
As is well known, the ignition timing of the internal combustion engine has an influence on an amount of the exhaust gas emitted and gas mileage. In order to realize an optimum ignition timing, it is conventional to use a distributor which rotates in synchronism with the internal combustion engine. The ignition timing is controlled by a centrifugal advance mechanism and a vacuum advance mechanism which are disposed in the distributor.
Since, however, both the advance mechanisms are of a mechanical control system they are inferior in precision and responsiveness. In an attempt to eliminating these disadvantages, an electronic control system has been developed. In case of the electronic control system, various detectors are required to be provided for sensing various information delivered to the control system, and one of these detectors is an apparatus for detecting the rotational angle of a crank shaft.
A conventional apparatus for the detection of the rotational angle comprises a magnetic disc with a number of protrusions formed at the outer periphery thereof and one or more pick up coils disposed in opposition to the protrusions with an appropriate spacing. The disc is mounted to rotate in synchronism with the crank shaft so that an amount of magnetic flux intersecting each of the pick up coils may change in the form of an alternating wave. As a result, an amount of the rotational angle can be sensed as the number of pulses obtained by shaping the alternating signal of the pick-up coil.
Another type of a conventional detector consists of a rotary disc having a number of apertures provided along the periphery thereof and a photoelectric transducer which is disposed to face the disc so as to produce an alternating signal changing in accordance with the rotation of the disc.
The rotational angle detector is usually required to generate two kinds of pulses. One is an angle signal which is to be generated at each time the crank shaft is rotated by an amount of a certain angle. This angle is determined by the number of the protrusions disposed at equal intervals along the outer periphery of the rotary disc. The other is a reference signal which is provided in correspondence with the number of cylinders of the engine. For example, in case of a 4-cycle 6 cylinder engine, a reference signal must be generated at an angular interval of 120.degree. in a revolution. For the control of the ignition timing, these two kinds of pulses must be generated in synchronism with each other. To this end, first protrusion for the generation of the angle signal and second protrusions for the reference signal have been formed on the same disc. In this case, however, the first and second protrusions should be located at a considerable distance in the thickness direction of the rotary disc, i.e., the axial direction of the crank shaft. Otherwise, a relatively high level of noise will be superposed on the reference signal because of an undesirable magnetic coupling between the pick-up coil for the reference signal and the angle signal-generating protrusion. Therefore a reference signal detector circuit at the succeeding stage will operate erroneously. However, a cooling fan, an air cooling compressor, an air pump for purifying the exhaust gas and the like are coupled to a pulley which is directly coupled with the crank shaft. Due to restriction of the mounting space of these components, it has been very difficult to dispose both the protrusions for the reference signal and those for the angle signal at a considerable distance in the thickness direction of the rotary disc.
The conventional detector has another drawback as follows. It is desirable from the viewpoint of high accuracy of the angle detection to provide as many protrusions as possible at the outer periphery of the disc thereby producing the angle signal of a high frequency. Due to the restriction of the mounting space in the vicinity of the crank shaft, however, the rotary disc is also restricted in its diameter. Therefore, it has been difficult to increase the number of the protrusions of the disc and thus the detection precision has been inferior.